The present invention relates to a method and an apparatus for improving bit-images in printing and video display devices, and more particularly to a method and an apparatus for thinning bitmap images with parallel bit-block transfer operations.
Previously, there have been several methods disclosed to improve bit-image quality. For example, in U.S. Pat. No. 4,517,604 to Lasher et al., a method for reducing line width variations in bilevel video images is described. In this method, print element (pel) data is scanned into two arrays. The first array is scanned row by row to identify any black or white pel runs. White or black pels in the second array are overwritten with gray pels as a function of the pel runs identified in the first array. The method is then repeated column by column. The resulting second array will have reduced line width variations than in the first array.
In U.S. Pat. No. 4,531,120 to Brownwell, Jr. et al., a method for superposing one graphic pattern on another is described. A computer is programmed to produce contoured intermediate arrays for a dominant graphic. These arrays are used to mask the array of an inferior graphic to produce a partial array. The partial array is DOT ORed with the dominant graphic to produce superposed graphics separated by a background space contoured to the dominant graphic.
In U.S. Pat. No. 4,688,031 to Haggerty, a method of converting color images to monochromatic images is described. In this method, the original background of a character is transformed into a monochromatic color pattern mask and the foreground is transformed into a dark field if the background color is white, or the foreground is transformed into a white field if the background color is dark.
In U.S. Pat. No. 4,780,711 to Doumas, an assumed boundary lines method is shown. In this method, an array of pixels in an image is selected and compared to a plurality of predetermined pixel array patterns. When a match is found, an assumed contour line is determined running through the array. The intensity of the center pixel of the array is chosen based on the angle of the assumed line through the array.
In U.S. Pat. No. 4,851,825 to Naiman, a grayscale character generator and method is shown. A master array is convolved with a filter array to determine the placement of gray pixels. The placement of a gray pixel in a certain area is determined by analyzing pixel data around this area using rectangular windows.
A method for the removal of jagged edges in diagonal lines is a process called spatial filtering. In this process, a processor is used to simulate a flying spot scanner. This type of scanner employs a moving spot of light that passes over a document and detects the intensity of the light reflected from the document. As an alternative, the source image can be effectively resampled with each bit in the output being the average value of the area illuminated by a "spot" at a given time. The "spot" is simulated by a spatial filter which comprises a set of values for each bit of the filter. These values can be either identical or computed according to a weighting function.
The filter is overlaid on the image at different areas. At each step, an output value is computed from the weighted average of the bits under the filter. This output value is then used to form a new bitmap for output to a device such as a xerographic processor. A problem with this method is that in font processing, for example, the spatial filtering may modify vertical and horizontal lines in characters. Raster scan output devices such as CRTs and printers print vertical and horizontal lines without aliasing, thus modifying these lines may cause a degradation of quality.
One of the deficiencies of the aforementioned methods and devices for improving bit-image quality is the amount of processing time and effort required for implementation. Another deficiency in previous methods is their complexity.